The present invention relates to a switching power supply unit, and more particularly to a synchronous rectifying switching power supply unit, a switching power supply unit having a half-bridge circuit, and a switching power supply unit using a plurality of converters connected in series.
(Related Art 1)
Conventionally, DC/DC converters are known as a switching power supply unit. A typical DC/DC converter converts an alternating current input to direct current once by using a switching circuit, then transforms (boosting or lowering) the voltage by using a transformer, and further converts the direct current to alternating current by using an output circuit, whereby alternating current output having voltage different from the input voltage can be obtained.
In some cases, switching elements such as transistors are employed in an output rectifier for use in the DC/DC converter, so that the switching elements may be synchronously controlled with the switching circuit on the input side. The DC/DC converter having such an output rectifier is generally called as a synchronous rectifying switching power supply unit.
FIG. 15 is a circuit diagram of a conventional synchronous rectifying switching power supply unit.
As shown in FIG. 15, the conventional switching power supply unit includes a transformer 1, a half-bridge circuit 2 provided on the primary side of the transformer 1, a rectifier circuit 3 provided on the secondary side of the transformer 1, a rectifier-transistor driving circuit 4 provided on the secondary side of the transformer 1, a smoothing circuit 5 provided at the following stage of the rectifier circuit 3, and a control circuit 9 for controlling on/off of a first main switch 7 and a second main switch 8 provided in the half-bridge circuit 2 based on the result of monitoring output voltage Vo via an insulating circuit 6.
The half-bridge circuit 2 includes a first input capacitor 11 and a second input capacitor 12 connected in series between both ends of an input power supply 10 in addition to the first and second main switches 7 and 8. The primary winding 20 of the transformer 1 is connected between a node where the first and second main switches 7 and 8 are joined and a node where the first and second input capacitors 11 and 12 are joined. The rectifier circuit 3 has a first rectifier transistor 13 and a second rectifier transistor 14. The drain of the first rectifier transistor 13 is connected to the first secondary winding 21 of the transformer 1, whereas the drain of the second rectifier transistor 14 is connected to the second secondary winding 22 of the transformer 1. As shown in FIG. 15, as the source of the first rectifier transistor 13 and the source of the second rectifier transistor 14 are short-circuited, a voltage waveform appearing between the common source node of both transistors and a node where the first and second secondary windings 21 and 22 of the transformer 1 are joined forms an output from the rectifier circuit 3. The rectifier-transistor driving circuit 4 has a first diode 15 connected between the gate and source of the second rectifier transistor 14 and a second diode 16 connected between the gate and source of the first rectifier transistor 13. The third secondary winding 23 of the transformer 1 is connected between the cathode of the first diode 15 and the cathode of the second diode 16. Further, the smoothing circuit 5 has a smoothing inductor 17 and a smoothing capacitor 18.
With the arrangement above, the first and second main switches 7 and 8 are turned on alternately under the control of the control circuit 9 at intervals of predetermined dead time, whereby the output voltage Vo determined by the input voltage Vin and the turn ratio of the transformer 1 is applied to a load 19.
FIG. 16 is a timing chart showing the operation of the conventional synchronous rectifying switching power supply unit. In FIG. 16, Vgs7 and Vgs8 mean the gate-source voltages of the first and second main switches 7 and 8 respectively; Vds13 and Vds14 mean the source-drain voltages of the first and second rectifier transistors 13 and 14 respectively; and Vgs13 and Vgs14 means the gate-source voltages of the first and second rectifier transistors 13 and 14 respectively.
As shown in FIG. 16, in the conventional synchronous rectifying switching power supply unit, the first and second main switches 7 and 8 are driven alternately under the control of the control circuit 9 at intervals of predetermined dead time, and in response to the operation, the secondary voltage is generated across the source and drain of the second rectifier transistor 14 during the interval the first main switch 7 is “on”, whereas the secondary voltage is generated across the source and drain of the first rectifier transistor 13 during the interval the second main switch 8 is “on”.
In this case, in the rectifier-transistor driving circuit 4, the first diode 15 is turned on during the interval the first main switch 7 is “on ” and the second diode 16 is turned on during the interval the second main switch 8 is “on ”. Consequently, during the interval the first main switch 7 is “on ”, the gate-source channel of the first rectifier transistor 13 is driven and turned on, and during the interval the second main switch 8 is “on ”, the gate-source channel of the second rectifier transistor 14 is driven and turned on. Further, as the gate of the first rectifier transistor 13 and the gate of the second rectifier transistor 14 are short-circuited via the third secondary winding 23 of the transformer 1 during the interval the first and second main switches 7 and 8 both are “off ”, the gate-source voltages of the first rectifier transistor 13 and the second rectifier transistor 14 each become intermediate voltages.
As the first rectifier transistor 13 is turned on during the whole interval the second main switch 8 is “off ” and as the second rectifier transistor 14 is turned on during the whole interval the first main switch 7 is “off ”, no current is practically allowed to flow into the body diode of the first rectifier transistor 13 and the body diode of the second rectifier transistor 14, so that rectification can be carried out with a small loss.
(Related Art 2)
There have been proposed so-called two-stage converters for electronic systems like computers and as one example of a switching power supply unit for efficiently and stably supplying voltage, a preceding-stage buck converter and a following-stage half-bridge converter are combined in such a two-stage converter.
The buck converter is used for stepping down input voltage to a certain voltage level, whereas the half-bridge converter employs a half-bridge circuit for converting the input voltage to AC voltage, insulating, rectifying and smoothing the AC voltage to generate DC voltage.
A rectifying-smoothing circuit comprises a self-drive type synchronous rectifying circuit formed with a synchronous rectifying switch element connected to the secondary winding side of a transformer, capacitors and an inductor.
As described in a document under the title of “Buck+Halfbridge (d=50%) Topology Applied to very Low Voltage Power Converters” by P. Alou, J. Oliver, J. A. Cobos, O. Garcia and J. Uceda in the IEEE Applied Power Electronics Conference (APEC), 2001, a two-stage converter arrangement is made through the steps of fixing to 50% the duty ratio of a main switch element provided in a following-stage half-bridge converter and controlling the duty ratio of a switching element provided in a preceding-stage buck converter so as to make the duty ratio of the switching element variable in accordance with output voltage.
(Related Art 3)
There has been proposed a technique recently for exciting the primary winding of a transformer by using a half-bridge circuit, wherein a buck converter circuit and the half-bridge circuit are connected in series as the primary circuit of a switching power supply unit; and the buck converter circuit is used to step down input voltage Vin and supply the input voltage thus stepped down to the half-bridge circuit (Buck+Half Bridge (d=50%) TopologyApplied to Very Low Voltage Power Converters, IEEE APEC, 2001, Session 19.4).
When these circuits above are used as the primary circuit of the switching power supply unit, control is exerted so that the duty of a switching element provided in the half-bridge circuit is fixed to a predetermined quantity and that the duty of a switching element provided in the buck converter circuit is set to a predetermined quantity according to output voltage Vo. As comparatively low voltage is thus obtainable efficiently and stably as the output voltage Vo, this switching power supply unit is most suitable usable as a power supply for computers, for example.
FIG. 17 is a circuit diagram of a conventional switching power supply unit having such a primary circuit as described above.
As shown in FIG. 17, the conventional switching power supply unit includes a transformer 51, a buck converter circuit 53 connected to an input power supply 52, a half-bridge circuit 54 that is connected to the buck converter circuit 53 and used for exciting the primary winding of the transformer 51, a rectifier circuit 55 provided on the secondary side of the transformer 51, a smoothing circuit 57 provided at the following stage of the rectifier circuit 55 and connected to a load 56, and a control circuit 63 for monitoring output voltage Vo via an insulating circuit 58 and performing on/off control over a first and a second main switch 59 and 60 provided in the buck converter circuit 53 according to the monitored result and performing on/off control over a third and a fourth main switch 61 and 62 provided in the half-bridge circuit 54.
The buck converter circuit 53 has an inductor 64 in addition to the first and second main switches 59 and 60; the half-bridge circuit 54 has a first and a second input capacitor 65 and 66 connected in series across the output terminal of the buck converter circuit 53 in addition to the third and fourth main switches 61 and 62; and the primary winding of the transformer 51 is connected between a node where the third and fourth main switches 61 and 62 are joined and a node where the first and second input capacitors 65 and 66 are joined. Further, the rectifier circuit 55 has a first and a second diode 67 and 68; and the smoothing circuit 57 has a smoothing inductor 69 and a smoothing capacitor 70. The rectifier circuit 55 and the smoothing circuit 57 constitute an output circuit.
With the arrangement above, the first and second main switches 59 and 60 provided in the buck converter circuit 53 are alternately turned on with predetermined dead time held therebetween under control of the control circuit 63, whereby the constant internal voltage Vin2 determined by the duties of input voltage Vin1 and the first and second main switches 59 and 60 appears across the output terminal of the buck converter circuit 53. On the other hand, the third and fourth main switches 61 and 62 provided in the half-bridge circuit 54 are alternately turned on/off with a predetermined quantity of duty under control of the control circuit 63. Thus, the constant output voltage Vo determined by the internal voltage Vin2 and the turn ratio of the transformer 51 is given across the load 56.
Regarding the first related art, what has been described above refers to ideal operation, and in actual circuits, there unavoidably occurs a slight delay in the timing of operations of the first rectifier transistor 13 and the second rectifier transistor 14. Ideally, the first rectifier transistor 13 is turned off simultaneously at the timing (time t0) the secondary voltage is generated across the source and drain of the secondary rectifier transistor 13, and the second rectifier transistor 14 is turned off simultaneously at the timing (time t1) the secondary voltage is generated across the source and drain of the secondary rectifier transistor 14. Actually, however, the timing the first rectifier transistor 13 is turned off slightly delays behind the time t0 and the timing the second rectifier transistor 14 is turned off slightly delays behind the time t1.
For the reason above, a through current flows into the first rectifier transistor 13 in a brief interval of time after the secondary voltage is generated across the source and drain of the first rectifier transistor 13, and similarly, through current flows into the second rectifier transistor 14 in a brief interval of time after the secondary voltage is generated across the source and drain of the second rectifier transistor 14. The through currents result in power loss and the problem is that the lowering of conversion efficiency is caused to the whole switching power supply unit.
In the two-stage converter as described in the second related art, current flowing through a synchronous rectifying switch element on the secondary winding side of a transformer is caused to have a commutation period due to the leakage inductance of the transformer provided in the half-bridge circuit and then voltage is generated across both ends of the synchronous rectifying switch element.
In case where the commutation period is longer than delay in the operation of the synchronous rectifying switch element (turn on/off period), through current flows as the synchronous rectifying switch elements are simultaneously turned on and the synchronous rectifying switch elements may be damaged when the worst comes to the worst.
Particularly in the case of a low ON resistant synchronous rectifying switch element, operation-delay time tends to become longer, this phenomenon appears conspicuously.
Although this problem can be dealt with by coarsely coupling the transformer in order to increase the leakage inductance and prolong the commutation period. However, power loss may increase caused by the increase of the interval that the synchronous rectifying switch element cannot be turned on, and further, there may be brought about a bad influence resulting from an increase in loss because of the leakage inductance and spike noise.
The synchronous rectifying switch elements can be prevented from being simultaneously turned on by adding to the half-bridge circuit a drive timing circuit for controlling the timing that the synchronous rectifying switch element is operated. However, the problem in this case is that a switching power supply unit tends to become large-sized accompanied with an increase in cost as the number of parts increases.
Regarding the third related art, a user may be requested to be able to switch the values of the output voltage Vo in order to have different kinds of loads driven by one kind of switching power supply unit. In case where the user is allowed to switch the output voltage Vo between 3.3V and 1.5V, a step-down range to be covered by the buck converter circuit 3 as the first stage converter grows larger, and the load of the buck converter circuit 3 is heavy when lower voltage (e.g., 1.5V) is required as the output voltage Vo, and the problem is that loss tends to increase.
In case where a lower value of the output voltage Vo is set by the user, the output voltage Vo can be lowered by reducing not only the duty of the buck converter circuit 3 but also the duty of the half-bridge circuit 4 as a second stage converter. In this case, however, the stability of the output voltage Vo may be ruined because a plurality of converters operate to stabilize the output voltage Vo. In order to prevent the stability of the output voltage Vo from being ruined, the converter-to-converter operation needs to be properly regulated, which results in complicating the control operation. Particularly when transistors as rectifying elements constituting a rectifier circuit are used and turned on/off by utilizing the secondary voltage of the transformer 1, the loss produced in the rectifier circuit tends to increase because the dead time of the half-bridge circuit 4 fluctuates as the duty of the half-bridge circuit 4 fluctuates. In case where the duty of the half-bridge circuit 4 is lowered in response to a demand for lower voltage (e.g., 1.5V) as the output voltage Vo, the dead time of the half-bridge circuit 4 increases, whereby the interval during which no voltage is generated on the secondary side of the transformer 1 becomes longer. Consequently, as the conducting period of the rectifier transistors constituting the rectifier circuit becomes shortened, current is allowed to flow into the body diodes over a long period of time.